1. Field of the Invention
The present invention relates to vacuum pumps for use, for example, with semiconductor manufacturing apparatus and, more particularly, to a vacuum pump capable of absorbing and reducing damaging torque when abnormal torque is generated in the pump.
2. Description of the Related Art
During a semiconductor manufacturing operation, a conventional vacuum pump as shown in FIG. 4 is known and used during various process, such as a dry etching process, to exhaust gas from a high-vacuum process chamber for producing a high vacuum.
The vacuum pump of FIG. 4 has a rotor 2 which is rotatably arranged inside an outer casing 1 that connects a cylindrical base member 3 and a cylindrical pump case 4, wherein a blade structure consists of multistage rotor blades 9 on the upper outer periphery of the rotor 2 and multistage stator blades 10 arranged alternately with the rotor blades 9 and functions as a turbo molecular pump by the rotation of the rotor 2, and a spacing structure constituted by the lower outer periphery of the rotor 2 and a thread groove 12 formed in the inner peripheral portion of the base member 3 which opposes thereto functions as a thread groove pump by the rotation of the rotor 2.
With such conventional vacuum pump, however, the rotor 2 may be broken due to stress concentration on the rotor 2 depending on the use conditions. When such a breakage occurs during high speed rotation, the rotation balance of the entire rotation body constituted by the rotor blades 9 and the rotor 2 is lost immediately. Accordingly, the rotor blades 9 may be brought into contact with the inner periphery of the pump case 4 or the lower periphery of the rotor 2 may collide with the inner peripheral portion of the base member 3 to produce damaging torque that applies circumferential torsional rotation to the entire outer casing 1 composed of the pump case 4 and the base member 3, which may break a process chamber 14 or fastening bolts that fasten the pump case 4 to the process chamber 14.